as it was feared that the trout might become 

 less resistant in the still water . 



One to 14-day TL/m determinations 

 for 14-day trout fry with yolk sacs: -- Four- 

 teen-day-old trout fry obtained from the 

 hatchery two days prior to the experiment 

 were exposed to serial dilutions of DDT in the 

 following concentrations: 10, 3.2, 1.0, .56, 

 .32, and . 18 ppm DDT. Five fry were used 

 for each concentration and were placed in 3-1/2 

 liters of the DDT treated water. Museum jars 

 served as containers. The control consisted 

 of five fish in untreated well water. Observa- 

 tions on percent survival and general physical 

 reactions were recorded and dead fish removed 

 daily . At the end of two weeks, the surviving 

 trout fry were fixed in Bouin's fixative and pre- 

 pared for histological examination. 



One to 14 -day TL/m determinations 

 for 10 -week trout fry: -- Ten-week -old trout 

 fry, pigmented and without yolk sacs, were 

 tested under conditions identical to those ap- 

 plied to the 2 -week fry. The concentrations 

 tested were .0056, .0032, .0018, .0010, 

 .00056 ppm with a control of untreated well 

 water . The trout had been kept in aquaria in 

 the laboratory for 7 weeks before the experi- 

 ment, had been fed medium sized tropical fish 

 food since the fourth week, and were fed daily 

 during the bioassay. 



One to 14-day TL/m determinations 

 for number one fingerlings: - - A bioassay on 

 11 -week -old fish, averaging approximately 

 3.3 cm in length, was conducted two days 

 after the fish arrived from the local hatchery. 

 Conditions were similar to those in the previous 

 trout assays except that 10 fish were used for 

 each concentration instead of five and were kept 

 in 2 -gallon "squash" type aquaria. The finger - 

 lings were regularly fed food pellets obtained 

 from the hatchery. The concentrations tested 

 were .032, .010, .0032, .0010, and .00032 

 ppm. 



Histological study of trout fry and adult guppy 

 tissues following exposure to DDT 



Hematoxylin -eosin stained paraffin 

 sections of Bouin's-fixed tissue cut at 8u were 



made of the kidney, liver, intestine, and brain 

 of four male guppies, two of which were sacri- 

 ficed after 24 hours and two after 48 hours of 

 exposure to .032 ppm DDT. This concentra- 

 tion is toxic to adult guppies within 96 hours. 

 Similar preparations were made from two 

 normal guppies for controls . The fish were 

 fed during the exposure to DDT. Cross sec- 

 tions of trout fry surviving a 14-day bioassay 

 in 10, 3.2, and .56 ppm DDT, begun when the 

 fry were two weeks old, were also prepared for 

 histological study. Three normal trout of the 

 same age served as controls. 



RESULTS 



Bioassays w i th the guppy 



One to 14 -day TL/m determinations 

 for adult guppies: -- The TL/m values for 

 adult guppies over a 2 -week period were deter- 

 mined by graphical analysis to be .027, .0195, .0195, 

 and .018 ppm for 2, 4, 7, and 14 days, respec- 

 tively (table 1; refer to fig. 1 for the 14 day 

 determination). As .018 was the TL/m for 



Table 1: --Survival of adult giqipies In DDT solutions 



and TL/m detemixiatiors over a 2-week period. 



14-day exposure and extrapolation of a time 



concentration curve of tolerance (fig. 2, form 

 recommended by Hart^al_., 1945) indicated 

 that concentrations below this amount might be 

 tolerated for an indefinite time, .0185 was the 

 highest concentration used in the long range 

 chronic exposure test. Symptoms of stress in 

 response to DDT were hyperactivity, muscular 

 convulsions, slight paling of pigmentation, fol- 

 lowed by loss of balance and death. The time 

 of onset of the symptoms varied with the con- 

 centration . 



Tolerance of .032 ppm DDT by two 

 different strains: --Placing a second strain 

 of fish in .032 ppm gave results that Indicated 

 that strains from varying backgrounds differ 



